1. Field of the Invention
The present invention relates to an electric driver unit for an electric valve timing adjusting apparatus that adjusts timing of opening and closing a valve of an internal combustion engine.
2. Description of Related Art
Internal combustion engines (referred to also as engine) for a vehicle are equipped with a valve timing adjusting apparatus, which adjusts timing of opening and closing a valve of the engine in accordance with a driving state of the engine. A valve timing adjusting apparatus adjusts a rotation phase of a camshaft relative to a crankshaft of the engine to an advanced angle or a retarded angle, and thereby changes the timing of opening and closing the valve in relation to rotation of the camshaft.
An electric valve timing adjusting apparatus driven by a motor is known as one type of valve timing adjusting apparatus. An electric valve timing adjusting apparatus employs a brushless motor capable of controlling the rotation phase with high accuracy. JP-2005-269875A corresponding to US patent application Ser. No. 2005/0206254 proposes an electric motor having a control circuit for driving the electric valve timing adjusting apparatus.
In general, a large current is required as a drive current of the motor for the electric valve timing adjusting apparatus because the large drive current can improve response performance of the electric valve timing adjusting apparatus. However, a flow of a large current to a circuit can bring a problem associated with heat generation. For example, since the circuit typically employs a MOSFET (metal-oxide-semiconductor field-effect transistor, which is one type of field effect transistors) as a switching element for switching voltage application direction, there arises a difficulty that the MOSFET generates a heat and the MOSFET may be damaged by the heat. When a circuit element operable at a large current is used, there arises another difficulty that a circuit board may increase in weight. For addressing the latter difficulty, JP-2006-37837A proposes a driver provided with a laminating structure of a circuit board for a control circuit.
From a viewpoint of suppressing the heat generation in a MOSFET, the drive current is conventionally controlled and maintained at a constant value by cutting power distribution to the motor. More specifically, the power distribution to the motor is temporarily cut when a value of a current flowing through a shunt resistor reaches a target current value. When the value of the current is decreased, the power is re-distributed to the motor. By repeatedly performing the above operations, the drive current is controlled and maintained at an approximately constant value so as not to exceed the target current value.
The inventors of the present application have however revealed that there is a difficulty in connection with the above described control of the drive current. For example, the response performance of the electric valve timing adjusting apparatus is worsened when an engine is cool. This is because the temperature of surrounding oil is a factor of worsening the response performance of the electric valve timing adjusting apparatus. After an engine is started, the response performance becomes low until the oil temperature rises above a certain level.